1. Field of the Invention
The present invention relates to a depth-of-field preview apparatus provided in a single lens reflex camera.
2. Description of the Prior Art
A single lens reflex camera is known and having a so-called full aperture light measurement system in which a diaphragm stop-down system is held at a full aperture setting position to maximize the brightness of a view-finder image to thereby facilitate focusing, and at the same time the brightness of an object is measured by a photometer on the camera body side and through the full aperture to determine an optimum aperture value, whereas at the time of photographing, the diaphragm stop-down system stops-down the diaphragm to the optimum aperture.
A camera system having such a diaphragm stop-down system usually incorporates therein a depth-of-field preview apparatus for enabling a camera operator to preview the depth of field, etc. through the field of view in the view-finder prior to photographing. By manually operating an operating portion of such a depth-of-field preview apparatus, a switch is changed over to its preview condition for causing an electromagnetic means to release the diaphragm stop-down system so that it can stop-down the diaphragm to a reduced aperture size, for example, a preset aperture size.
The diaphragm stop-down system if so arranged as to move back to an open condition in response to a releasing of the operating portion, that is when camera operator's finger is removed the aforesaid operating portion. This is very inconvenient for the camera operator since the operator must continue to manipulate the operating portion while he or she wishes to observe the depth-of-field.
For the purpose of overcoming the above inconvenience, there may be provided means for locking the diaphragm stop-down mechanism at its stopped down position.
Furthermore, for the purpose of informing the camera operator that the camera is under the preview condition, there may be also provided a display member which provides a display in response to the change-over of the above mentioned switch to the preview condition.
However, with a camera including the above mentioned display member and the diaphragm stop-down system locking means, the display is turned off when the camera operator releases the operating portion, while the camera is maintained in the preview condition. Therefore, the camera operator may misunderstand that the camera is not in the preview condition after a releasing of the operating portion.